Use of air activated gravity conveyors in a continuous particulate removal process from an ESP or baghouse

ABSTRACT

Disclosed is a method for transporting particulate material produced in an industrial process from a particulate collection means, through an underlying hopper, and into an air activated gravity conveyor system that is sized to convey particulate material at a rate that is at least about three times the rate that such particulate material is produced in the industrial process.

FIELD OF THE INVENTION

This invention relates to the continuous removal of particulate materialfrom an electrostatic precipitator or baghouse system that in turnremoves particulates from an industrial exhaust gas stream. Theinvention has particular relevance to the power plant industry but isnot limited to use in that field.

BACKGROUND OF THE INVENTION

The emission of particulates in industrial exhaust gas streams must becarefully controlled in light of federal, state, and local regulationsdesigned to curtail pollution. As one example, fly ash is a fineparticulate residue which is a by-product of the burning of powderedcoal collected from the flue gas stream of power plants and othercoal-burning installations, trash-to-energy facilities, steel mills,coke ovens, foundries, pulp and paper and co-generation plants. Tenpercent ash content is not unusual in some bituminous coals, so, forexample, a power plant burning 10,000 tons of this coal a day willtypically produce 1,000 tons of ash, 800 tons of which is typically flyash which is carried off in a flue gas stream and 200 tons is bottomash. Larger power plants may produce more than 3000 tons of fly ash in agiven day.

A utility power plant system typically comprises a boiler for burningcoal to produce heat used to generate electricity. The boiler producesnon-combustible materials that exit the boiler in the form of gaseswhere they pass to an ash disengagement system that is coupled to theboiler that receives the gases exiting the boiler and separates andcollects most of the ash contained within the gases. A fly ash transportsystem is coupled to the ash disengagement system for receiving thecollected ash and transporting the collected ash to a remote storagevessel typically via a pneumatic conveying system.

The Clean Air Act (CAA) requires that facilities burning fuels thatproduce fly ash must remove over 99 percent of the particulate matterfrom the exhaust gas prior to its release into the atmosphere. The EPAcan levy heavy fines for non-compliance as well as shut down facilitiesuntil corrective action takes place.

Generally, two methods are used to separate fly ash from flue gas. Themore common in a typical power plant operation is an electrostaticprecipitator (ESP), which consists of a charged grid that the flue gaspasses through. As the flue gas passes through the grid, the fly ashparticles become charged and adhere to collection electrodes. At apredetermined interval, an automatic hammer raps the electrodes,loosening the fly ash and allowing it to fall by gravity and collect ina hopper located underneath the ESP. The ash is removal from the hoppergenerally in a predetermined sequence. Failure to remove the ash in atimely manner can cause the ash to short out the electrostatic grid,allowing the fly ash to vent with the flue gas, resulting in a CAAcompliance violation.

The second method of separating fly ash from flue gas is a bag house.This system consists of multiple bag house compartments, each containingan array of fabric bags that will be used to capture the fly ash as theflue gas passes through the filter bags. Periodically, each compartmentwill be cleaned by pulsing the bags to dislodge particulates into a flyash hopper beneath the compartment. As with electrostatic precipitators,timely removal of fly ash is critical. If the level rises to a pointwhere it reaches the filters, they become clogged and require a manualcleanout. The weight of the ash can damage the bags, causing tears andresulting in a noncompliant release of ash. Therefore, level measurementdevices are necessary in hoppers used in both ESP and bag houseapplications to avoid non-compliance fines and unscheduled shut downs aswell as to prevent costly repairs to precipitators and bag houses.

Power plants, for example, will employ a plurality of hoppers under anESP(s) or fabric filter bags. Generally the hoppers will beautomatically emptied sequentially. Typically the fly ash removed fromthe collection hopper is conveyed to a remote storage or disposal siteby a pneumatic conveying system. The hoppers will employ a shut offvalve having automatic controls that will open during the predetermineddischarge time and will close after the hopper is emptied.

During the interval between when the hoppers are emptied, small amountsof fly ash will fall unimpeded from the ESP plates or bags into theunderlying hopper. Collected fly ash, like other particulates, is a hot(>300° F.), dusty, abrasive material that is often sticky causing it tobecome cohesive and coat everything it contacts. If it is allowed to sittoo long in a hopper it can plug or bridge the hopper and impede theprocess of emptying the hopper. At best this can require undoing theplug manually. At worse it can damage the ESP or bag and perhaps causeenvironmental problems.

It is an object of the invention to devise an apparatus and process forremoving particulates from ESP or fabric filter hoppers in a manner thatreduces the likelihood of hoppers overfilling or having the particulatescause plugs within the hoppers.

SUMMARY OF THE INVENTION

The above and other objects are achieved by the apparatus and process ofthe present system in which there is no predetermined discharge time forthe hoppers. The hoppers are kept open to feed, for example, fly ashcontinuously into an air activated gravity conveyor system which carriesthe fly ash into a conveyor such as screw pump line charger from whichthe fly ash is injected into a pressurized convey pipeline whichtransports the material to storage or a disposal site.

It is a feature of the present invention that the air activated gravityconveyor system utilized in the present invention is, in the aggregate,sized to convey the particulate material collected by a plant's ESP orbaghouse at a rate at least about three times, and preferably betweenfrom about three times to about six times, and most preferably betweenfrom about three times to about five times, the rate that suchparticulate material is produced by the plant.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in connection with the annexed drawingswherein:

FIG. 1 is a side view partially in cutaway depicting a pair of adjacentfly ash hoppers, labeled 1 and 2.

FIG. 2 is a second view showing the same hoppers depicted FIG. 1 at alater period in time.

Similar numerals are utilized in the drawings to designate similarcomponents. The figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in detail in the context of a fly ashremoval process in a power plant.

Flue gas exiting a boiler in a power plant passes to a compartmentcontaining an ash disengagement system that typically comprises one ormore ESPs or fabric filter bags. With reference, for example, to a plantthat uses ESPs, an ESP in a power plant will empty into a plurality ofcollection hoppers. While every plant requires its unique solution, a600 MW generating unit may, for example, have an ESP which feeds into 48hoppers, which may be arranged in six rows with eight hoppers in eachrow. A first row will be adjacent to gas inlet of the compartment and alast row will be adjacent to the gas outlet of the compartment. Hoppersclosest to the gas inlet will tend to accumulate the largest amount offly ash, with each succeeding row removing less fly ash. In a bag housethe fabric filter bag compartments, and the underlying hoppers, aresimilarly arranged in a series of rows.

In prior art systems typically the ESPs or fabric bags are emptiedsequentially. The underlying hoppers will empty hopper by hoppergenerally at predetermined time periods or when they reach a certaincapacity, as determined by level switches or similar sensing devices.The emptying of the hoppers may not necessarily always be in sync withthe emptying of the ESPs or hoppers from which they are fed fly ash.

In the present invention there is no change from prior art methods ofhow the ESPs or fabric bags are still emptied, however the presentinvention differs from standard prior art methods in that the underlyinghoppers empty continuously into an adjacent air activated gravityconveyor, with a hopper displaying the heaviest flow of particulatematter into the air activated gravity conveyor at the same time that itsassociated ESP or fabric bag is hammered or pulsed cleaned.

FIG. 2 is a side view partially in cutaway depicting a pair of adjacentfly ash hoppers, numbered 1 and 2. For the purposes of the illustrationit does not matter whether the particular hoppers depicted are fed flyash from an ESP or a fabric filter bag. Hopper 1 is example of a hopperbeing in the first row of hoppers closest to a gas inlet of a boiler.Hopper 2 is an example of a hopper being in a second row of hoppers.FIG. 1 depicts a condition at the time that the ESP or filter bag (notshown) associated with hopper 1 is being emptied. FIG. 2 depicts acondition directly after the events depicted in FIG. 1, when the ESP orfilter bag associated with hopper 2 is emptied.

Hoppers 1 and 2 discharge fly ash into conventional air activatedgravity conveyor 3. Air activated gravity conveyors are well known inthe art and comprise a gas-permeable medium over which the material tobe conveyed is adapted to flow. Immediately below the gas-permeablemedium there is situated a plenum chamber through which air passesupwardly through the gas-permeable medium into the material. Thisaeration of the material fluidizes it and causes it to take onpseudo-liquid properties so that it will flow by gravity along the uppersurface of the gas-permeable surface. Air activated gravity conveyingsystems are well known in the art and include FLSmidth's Airslide™ airactivated gravity conveying system.

Typically conveying systems in traditional fly ash or other particulateremoval systems that air feed from a particulate collector such as anESP or baghouse are sized according to the particulate production rateof the plant. In a power plant the amount of fly ash produced will bedependant on the rate of coal utilization and the grade of coal beingutilized. It is a feature of the present invention that the airactivated gravity conveyor system utilized in the present invention issignificantly oversized compared to the particulate production rate ofthe plant. The air activated gravity conveyor system is designed tocarry the particulates collected in the hoppers to the conveyingapparatus at a minimum rate, in the aggregate, of at least about 300% ofthe expected rate of accumulation in the hoppers, or a minimum of atleast three times the normal fly ash creation rate of the plant, in amanner that there will be no accumulation of ash in the hoppers undernormal or unusual operating conditions, and such that all hoppers willremain substantially empty during all operating conditions. Therefore,in the example set forth above in which a power plant produces 800 tonsof fly ash a day (or 33⅓ tons per hour when operating 24 hours/day), theair activated gravity conveying system will be sized to remove at least100 tons of fly ash per hour.

Preferably, the air activated gravity conveyor system utilized in thepresent invention is, in the aggregate, sized to convey the fly ash at arate of between from about three times to about six times, and mostpreferably between from about three times to about five times, the ratethat particulate material removed by a ESP or baghouse system isproduced by a plant.

It is also a feature of the particulate removal system of the presentinvention that a standard valve adaptable to being automatically openedand closed is not a necessary component of the hopper. Rather gate 4 canbe utilized to manually close the hopper for safety purposes whenmaintenance work is needed on the system. While the system is inoperation gate 4 will always open permitted constant flow from ofmaterial from the hopper.

Referring again to FIG. 1, the flow 1 a of fly ash into air activatedgravity conveying line 3 will be particularly heavy because the ESP orfabric filter immediate above hopper 1 is being emptied, and fly ashwill flow directly through hopper 1 into conveying line 3. At the sametime there is a light flow 1 b of fly ash from hopper 2 into conveyingline 3. This represents a light flow of fly ash from the ESP or fabricfilter into hopper 2, fly ash that in conventional systems might residein the hopper for several hours and consequently can plug the hopper. Inthe present system fly ash does not reside in a hopper for anysubstantial period.

FIG. 2 represents the time period directly after the period in which theESP or fabric bag above hopper 1 has been emptied. FIG. 2 depicts thecondition when an ESP or fabric bag is emptied into hopper 2 andtherefore the flow 2 b of material from hopper 2 into air activatedgravity conveying line 3 is particularly heavy. Since the ESP or bagfeeding into hopper 1 was just previously emptied, there will besubstantially lighter flow 2 a of material from hopper 1 into airactivated gravity conveying line 3.

Air activated gravity conveyors 3 are typically arranged in a network,consisting of one entry point into the conveyor for each hopper of anelectrostatic precipitator, bag house or other dust collector. Airactivated gravity conveyors 3 use the fluidization principle totransport fly ash collected in the mentioned hoppers to a conveyingapparatus, in a continuous manner during normal and unusual operatingconditions of higher than normal material production rate.

Air activated gravity conveyors 3 transport the material to an apparatussuch as, but not limited to, a rotary airlock, screw pump, pressuretank, vacuum pickup, or other such pneumatic transfer system; or a beltconveyor, screw conveyor, or other such mechanical transfer system fromwhich the material is conveyed to a storage or disposal means. Thepreferred conveying apparatus is a screw pump that permits continuousoperation. Such pumps act as a screw type volumetric line charger thatuse a material seal and can introduce material into a pressure conveyingsystem. An example of suitable commercially available screw pumps isFLSmidth's Fuller-Kinyon™ pump. Preferably the conveying apparatus willbe sized to handle at least 200% of the ash flow production rate.

Typically, the air activated gravity conveyor network will converge thefly ash to a common flow valve that feeds into the conveying apparatus.Optionally two conveying apparatuses can be employed with one apparatusacting in a standby capacity. A manual cutoff gate valve may be deployedat the inlet of each conveying device to select or isolate a device asdesired. The rotary flow control valve will meter the fly ash into theinlet of the conveying device at a controlled rate for optimum ashconveyance under unusual conditions.

The present method is advantageous in that the equipment cost is lessthan a standard system, as, for example, cycling sequencing valves forthe hoppers are not needed, and the overall control system does not haveto be as extensive as in the prior art system that utilized vacuum orpressure conveying systems from the point of entry of the fly ash intothe hoppers. The present system also replaces expensive vacuum orpressure systems located at the material outlet of each hopper with acomparatively low cost and low maintenance air activated gravityconveyor system.

A further advantage of the present system is that the continuous flow ofash from the hopper does not give the ash opportunity to come to restand build up inside the hopper. Therefore conditions in which the ashwill bridge over in the hopper and fail to flow when the sequentialvalve is closed are virtually eliminated.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,deletions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as limited by theforegoing description but is only limited by the scope of the appendedclaims.

1. A method for transporting particulate material produced by a plant, said material having been collected from the plant's exhaust gas stream by a particulate collection means having an exit for the particulate material that is flow connected to an underlying hopper; said method comprising (i) directing particulate material from said particulate collection means into said hopper; (ii) directing particulate material from said hopper into an air activated gravity conveyor system flow connected to said hopper, wherein said air activated gravity conveyor system is sized to convey particulate material at a rate that is at least about three times the rate that such particulate material is produced by the plant; and transporting said particulate material by said air activated gravity conveyor system to a conveying means from which it is transferred to an end location.
 2. The method of claim 1 wherein said air activated gravity conveyor system is sized to convey particulate material at a rate that ranges from about three times to about six times the rate that such particulate material is produced by the plant.
 3. The method of claim 2 wherein said air activated gravity conveyor system is sized to convey particulate material at a rate that ranges from about three times to about five times the rate that such particulate material is produced by the plant.
 4. The method of claim 1 wherein the conveying means is a screw pump that feeds the particulate material into a pneumatic conveying line.
 5. The method of claim 1 wherein the conveying means is a mechanical conveying device.
 6. The method of claim 1 wherein the conveying means is sized to convey particulate material at a rate that is at least about 200% of the rate that such particulate material is produced by the plant.
 7. The method of claim 1 wherein the hoppers discharge particulate material continuously into the air activated gravity conveyor system.
 8. The method of claim 1 wherein the particulate material is fly ash.
 9. A method for transporting fly ash produced by a coal-burning boiler system, said fly ash having been collected from the system's flue gas stream by a particulate collection means having an exit for the fly ash flow connected to an underlying hopper; said method comprising directing fly ash that exits said particulate collection means into said hopper and thereafter continuously directing said fly ash from said hopper into an air activated gravity conveyor system flow connected to said hopper, wherein said fluidized air conveyor system is sized to convey fly ash at a rate that is at least about three times the rate that such fly ash is produced by the coal-burning boiler system; and transporting said fly ash by said air activated gravity conveyor system to a screw pump from which it is transferred to an end location, said screw pump being sized to convey fly ash at a rate that is at least about 200% of the rate that such fly ash is produced by the coal-burning system.
 10. The method of claim 9 wherein said air activated gravity conveyor system is sized to convey fly ash at a rate that ranges from about three times to about six times the rate that fly ash is produced by the boiler system.
 11. The method of claim 9 wherein said air activated gravity conveyor system is sized to convey fly ash at a rate that ranges from about three times to about five times the rate that such fly ash is produced by the boiler system.
 12. A fly ash transport system for transporting fly ash produced by a coal-burning plant, said system comprising (i) a particulate collection means for removing fly ash from the plant's flue gas stream and having an exit through which fly ash is discharged; (ii) at least one hopper disposed to receive fly ash that is discharged from said particulate collection means, said at least one hopper having a fly ash outlet; (iii) an air activated gravity conveyor system for conveying the fly ash, said conveyor system being disposed to receive fly ash from the fly ash outlet of said at least one hopper, wherein said air conveyor system is sized to convey fly ash at a rate that is at least about three times the rate that such fly ash is produced by the coal-burning plant; and (iv) a conveying means for receiving the fly ash from the air activated gravity conveyor system and conveying it to an end point.
 13. The transport system of claim 12 wherein said air activated gravity conveyor system is sized to convey fly ash at a rate that ranges from about three times to about six times the rate that such fly ash is produced by the plant.
 14. The transport system of claim 13 wherein said air activated gravity conveyor system is sized to convey fly ash at a rate that ranges from about three times to about five times the rate that such fly ash is produced by the plant.
 15. The transport system of claim 12 wherein the conveying means is a screw pump that feeds the fly ash into a pneumatic conveying line.
 16. The transport system of claim 12 wherein the conveying means is a mechanical conveying device.
 17. The transport system of claim 12 wherein the conveying means is sized to convey the fly ash at a rate that is at least about 200% of the rate that such fly ash is produced by the coal-burning plant
 18. The transport system of claim 12 further comprising means to enable the fly ash to flow continuously from the collection means through said hopper and into the air activated gravity conveyor system. 